HLA Class II cell surface molecules have important roles in cell interactions involved in immune responses, and specific alleles of Class II antigens appear to confer susceptibility to certain diseases, probably by virtue of their immune functions. HLA Class II genes are regulated in a complex manner, being constitutively expressed, inducibly expressed, or not expressed, depending on the cell type examined and its stage of differentiation. This application proposes various molecular and genetic approaches to study Class II gene regulation. Somatic cell hybrids of HLA Class II expressing and non-expressing and inducible cell types and mutants will be made in various combinations to define by complementation analysis the number of factors, pathways or subunits by which Class II expression is regulation. Preliminary evidence from this laboratory suggests that their is a transactive positive regulator of Class II genes which acts transcriptionally. A major aim of this proposal is to characterize the putative regulator of Class II expression, and its site and mode of action. Assays for the proposed transactive regulator will be developed so that the regulator can be characterized. Isolated mutants with low levels of a single Class II gene mRNA will be studied to determine whether the lesions are transcriptional. Transcription defective Class II genes and the normal genes from which they derived will be cloned. The ability of the regulatory molecule to bind to the normal and transcription-defective mutant genes and to induce DNase I hypersensitivity will be determined. Genomic clones of mutant genes with transcriptional lesions will be partially sequenced to map the affected nucleotide positions. The other major goal of this application is to clone the gene coding for the Class II transactive regulator, to determine how its gene product operates, and to map the sequences it binds in to or around HLA Class II genes.